1. Field of the Invention
The present invention relates to a coating method and, more particularly, to a coating method in which a surface of a flexible support (hereinafter referred to as a "web") made of plastic film, paper, metal foil, or the like, is continuously and uniformly coated with a coating composition at a high speed by means of an extrusion coating apparatus.
2. Description of the Related Art
As the coating method for applying a coating composition such as a magnetic coating composition, a photographic light-sensitive coating composition, or the like, onto a surface of a web, conventionally, coating methods using an extrusion coating apparatus, a curtain flow coating apparatus, a blade doctor coating apparatus, a slide coat coating apparatus, etc. are generally known. Of these methods, a coating method using an extrusion coating apparatus is employed in various fields because uniform thin layer coating can be accomplished (see, for example, U.S. Pat. No. 4,681,062, Japanese Patent Postexamination Publication Nos. Hei-1-46186 and Sho-63-88080, Unexamined Japanese Patent Publication Nos. Hei-2-174965 and Hei-2-265672, etc.)
The extrusion coating apparatus is a coating apparatus in which a coating composition is continuously discharged from a slot 24 between a back edge and a doctor edge so as to be applied onto a surface of a web 7 which is stretched by support rolls 2 and 3 located upstream and downstream, respectively, of a coating head 21 and which runs continuously along a back edge surface 22 and a doctor edge surface 23, as shown in FIGS. 5 and 6. The extrusion coating apparatus has at least one slot.
In recent years, the high-density and multilayer structure of a magnetic recording layer has been developed in the field of magnetic recording media, or the like. With the advance of the magnetic recording media, reduction of coating thickness of the magnetic layer applied onto a web has been required in a process of producing such magnetic recording media. On the other hand, in order to improve the productivity, it is required to increase the coating speed for applying a coating composition onto a web. Furthermore, with the advance of improvements of such a web, a thin web made of polyethylene naphthalate, aramid, or the like, and having a thickness of not larger than 10 .mu.m has been used.
When a coating composition is applied onto such a thin web having a thickness of not larger than 10 .mu.m by means of a coating method using such an extrusion coating apparatus as described above, however, there is a problem in that stripe irregularity of about 0.5 .mu.m with the pitch of from the order of several mm to the order of ten and several mm occurs in the coating layer over the direction of the width of the web, or thickness irregularity occurs in the direction of the width of the coating layer periodically over the direction of the length of the web.
Particularly, such stripe irregularity and thickness irregularity occur easily in the case where the thickness of the web is not larger than 10 .mu.m, so that these irregularities have a detrimental influence on the electromagnetic conversion characteristics such as output, C/N ratio, etc. Accordingly, the coating condition is conventionally determined by trial and error, but the yield is poor so that production efficiency is bad and quality is unstable.
Therefore, the inventors of the present invention have diligently examined the cause of the occurrence of the aforementioned stripe irregularity. As a result, it has been discovered that because the web 7 is pressed against the coating head 1 by the support rolls 2 and 3 located at the upstream and downstream sides, respectively, of the coating head 21 at the time of coating, the thin web 7 is buckled in the direction of the width of the web between the coating head 21 and each of the support rolls 2 and 3 as shown in FIG. 5 so that the contacting force between the coating head 21 and the web 7 is not uniform to thereby bring about the occurrence of stripe irregularity. For this reason, such buckling of the web 7 occurs easily when the thickness of the web is very thin so as to be not thicker than 10 .mu.m.
It has been further discovered that, if the distance between the coating head 21 and each of the support rolls 2 and 3 is reduced in order to prevent the buckling of the web 7, the running web 7 is moved up and down by the deviation .delta. (only the deviation .delta. of the support roll 3 is shown in the drawing) of rotation caused by the eccentricity of each of the support rolls 2 and 3 as shown in FIG. 6 so that the web contact force on the coating head 21 is changed by the up and down movement of the web 7, and therefore the coating thickness of the coating layer changes so that the thickness irregularity occurs periodically over the direction of the length of the web 7.